Magnetic nanoparticles are useful for a wide range of applications including high-density data storage, magnetic fluid, MRI (magnetic resonance image) contrast agent, separation processes, and biomedicine. Surface modification of nanoparticles with synthetic or natural polymer not only improves the dispersity and the stability of the nanoparticles, but also can be used to create complex nanoparticles with magnetic and polymer functionality. Chitosan is a natural biopolymer capable of chelating heavy metal ions, and adsorbing anions through ion exchange. In addition to typical adsorption separation, chitosan has great potential applications in biomedicine by forming complex with drugs, enzymes, proteins or DNA.
H. Honda et al. (J. Ferment. Bioeng, 86, 191 (1998)) disclose a method for preparing chitosan-conjugated magnetic particles by adding chitosan during the coprecipitation process of preparing iron oxide magnetic nanoparticles and binding chitosan to the iron oxide magnetic nanoparticles through coupling agents and cross-linkers. However, since the resultant magnetic composites are either aggregated or unstable due to polymer cross-linking or physisorption, the particle sizes become above 100 nm, even larger than 1000 nm. Therefore, monodisperse chitosan-Fe3O4 complex nanoparticles with particle sizes less than 100 nm cannot be obtained by the conventional method.
The present inventors, in Taiwan patent application Ser. No. 92108178, disclose a cationic magnetic nano-adsorbent prepared by covalently binding polyacrylic acid to the surface of iron oxide magnetic nanoparticles. This nano-adsorbent possesses the advantages of high adsorption capacity, fast adsorption rate, and easy magnetically manipulation, but is not suitable for anionic substances and multivalent heavy metal ions.
Briefly, chitosan or composite particles thereof are typically used in microscale or submicroscale application. Although they may be prepared as particles with diameter of a few tens of nanometers by microemulsification, it is quite difficult to purify the resultant product.
Accordingly, there is a need for magnetic nanoparticles and a method for producing the same which improves upon the drawbacks associated with the conventional process, e.g., larger particle size, particle aggregation or instability, purification difficulties, or limited application range.